The present invention relates to a method and an apparatus to determine the alveolar opening and/or closing of a lung.
Such a method and such an apparatus are especially useful to optimally set the control variables of an artificial ventilator as both the alveolar opening and the alveolar closing are important parameters of an atelectatic (=partially collapsed) lung.
In German intensive care units (ICUs), approximately 8.000-10.000 are artificially ventilated each day. The ventilator control variables, such as airway pressure (Paw) and respiratory rate (RR), are usually chosen based on known standard procedures, but often left constant afterwards and not adapted to the changing needs of a specific patient.
Today, the success of artificial ventilation is evaluated by using arterial blood gas analysis during which the partial pressures of oxygen (paO2) and carbon dioxide (paCO2) are determined. However, quite often these values are measured only 1-4 times a day. Since a human performs about 20.000 breath strokes per day, it becomes obvious that such a low xe2x80x9csampling ratexe2x80x9d may not be sufficient to evaluate the status in critical and unstable patients.
Patients with an acute respiratoy distress syndrome (ARDS) usually belong to this group of critical patients. Despite all sucesses in intensive care medicine, ARDS still is a pathological state with a mortality of 50%. The basic patho-physiological mechanism is the lack of xe2x80x9csurfactantxe2x80x9d, a substance which reduces suface tension resulting in a collapse of major lung fractions and a dramatically reduced gas exchange area.
To prevent undesirable sequelae and consecutive multiorgan failure, one important goal of protective ventilator therapy should be a gentle and early xe2x80x9creopeningxe2x80x9d of the lung. Choosing the airway pressures properly has an important impact on this.
Through the identification of the alveolar opening and especially of the alveolar closing pressures, a distressed lung may be kept open by proper choice of the airway pressure. However, the manual determination of opening and closing pressures is arduous and time consuming. To use the present invention in clinical practice, an automatic, computerized strategy is strongly recommended.
Prior to citing known methods to identify a lung collapse, a basic introduction to artificial ventilation shall be given:
The major function of the lung is the gas exchange, i.e. providing sufficient O2 to the circulation and eliminating CO2 from the body. If a human is not capable to perform this gas exchange himself anymore, he must be ventilated artificially.
FIG. 1 shows the human bronchial tree and an enlargement of some human alveoles. As in spontaneous ventilation, during artificial ventilation fresh air must be transported via the conducting parts of the brochial tree into the respiratory zone of the lung. The gas exchange actually happens in the so called xe2x80x9calveolixe2x80x9d, grape-shaped structures with an average diameter of about 70 xcexcm which are located in the termial part of the bronchial tree.
During spontaneous ventilation, contraction of the diaphragm produces a subathmospheric pressure within the lung which causes air to be sucked into the lung. By contrst, in most modern forms of artificial ventilation a positive airway pressure is applied to the patient which presses air into the lung (xe2x80x9cexcess pressure ventilationxe2x80x9d).
There are two major forms of ventilatory support: assisted (=augmented) and mandatory (=controlled) artificial ventilation.
In augmented artificial ventilation, the activity of the patient is monitored, either by detecting inspiratory flows sufficient to trigger an artificial breath stroke or by allowing the patient to breathe on top of a basic mandatory ventilatory support. These ventilation modes are especially used during weaning from the ventilator. By contrast, controlled mechanical ventilation (i.e. artificial ventilation without spontaneous breathing activity) is usually applied to more severly ill patients in which complete control of the breathing is desirable or necessary.
There are two major forms of controlled mechanical ventilation, namely pressure- and volume-controlled ventilation.
During pressure-controlled ventilation, the airway pressure is kept at desired levels during inspiration as well as during expiration. The corresponding pressure levels may be named, xe2x80x9cpeak inspiratory pressurexe2x80x9d (PIP) and xe2x80x9cpositive end-expiratory pressurexe2x80x9d (PEEP). Note that the alveolar pressure Palv actually varies in between these two pressure levels. FIG. 2 illustrates the time course of airway and alveolar pressure during pressure controlled ventilation.
On the ventilator, several control variables must be adjusted according to the patient needs including the respiratory rate (RR) and the inspiration to expiration ratio (I/E). The following eqn. describe the relationships                               RR          =                                    1                                                T                  insp                                +                                  T                  exsp                                                      ·                          60              min                                      ⁢                  
                ⁢        and                            (        1        )                                          I          /          E                =                              T            insp                                T            exsp                                              (        2        )            
with Tinsp the inspiration and Texp the expiration time. The inspired and exhaled volume during quiet breathing is named tidal volume (VT). Assuming a stationary operation and no leakage in the breathing system, VT is given by                               V          T                =                                            ∫              0                              T                insp                                      ⁢                                                            V                  .                                atem                            ⁢                              ⅆ                t                                              =                                                    ∫                T                            insp                              T                exsp                                      ⁢                                                            V                  .                                atem                            ⁢                              ⅆ                t                                                                        (        3        )            
During volume-controlled ventilation, a konstant air flow is applied during inspiration while expiration occurs passively against a given PEEP. FIG. 3 illustrates the time course of airway pressure, alveolar pressure and air flow during volume-controlled ventilation.
Note that volume-controlled ventilation guarantees delivery of a certain tidal volume while pressure-controlled ventilation does not. For this reason, some clinicians still prefer this form of mandatory ventilation. However, depending on the actual lung condition there are major disadvantages. In patients with a stiff lung, for example, Palv may reach undesirable limits and cause barotrauma. Furthermore, due to lung inhomogenities, local lung air flows may arise (so called xe2x80x9cPendelluftxe2x80x9d).
From Leonhardt, S., Bxc3x6hm, S. and Lachmann, B. xe2x80x9cOptimierung der Beatmung beim akuten Lungenversagen durch Identifikation physiologischer Kenngroessenxe2x80x9d, Automatisierungstechnik (at), Vol. 46, No. 11, pp 532-539, 1998 as well as from U.S. Pat. Nos. 5,660,170, 5,738,090 and 5,752,509, it is known that the airway pressures required to open or close a specific lung can generally be identified from measurements of the arterial oxygen partial pressure (paO2). After the identification procedure, the authors suggest to ventilate above the closing pressure.
It is known that both the identification and the later selection of ventilator parameters for long-term ventilation can be accomplished automatically by using a computer. A major disadvantage of this known method is that the measurement of this physiological parameter requires expensive and very sensible catheter systems and introduce possible damage to the patient (infections, bleeding, etc.).
Object of the present invention is to automatically provide a setting of ventilator parameters in critical patients.
This object is solved by a method according to the claims 1, 5 and 6 as well as an apparatus according to the claims 9, 11 and 12. By using the new feedback signals as claimed in this invention, in claim 13 an apparatus is presented aiming at automatic protective artificial ventilation of human lungs.
The invention is based on the cognition that the hemoglobin oxygen saturation (SO2), the endtidal CO2 concentration (etCO2) and the CO2 output (the elimination of CO2 volume from the body per unit time) can easily be obtained noninvasively and may be used, either solely or combined as parameters to identify the alveolar opening and closing pressure levels of the lung. An invasive arterial line is not necessary anymore. All three parameters may be measured outside the body and may well be used as feedback signals for automatic artificial ventilation.
Similar to using the arterial oxygen partial pressure (PaO2) as a parameter to identify alveolar opening or closing pressures, SO2 may be used for this task as well. For example, SO2 may be set to e.g. 80% by adjusting the ventilator in a proper way (e.g. adjust the inspiratory oxygen fraction fiO2). An alveolar opening due to a subsequent increase of ventilation pressure may then be detected by a large increase in SO2. Similiarly, an alveolar collapse due to a reduction of ventilation pressure may indeed be detected by a decrease in SO2.
However, using SO2 directly has the disadvantage that the saturation may temporaily reach rather low values which could cause life threatening situations.
Thus, a related object of this invention is to provide a method in which SO2 is feedback controlled to stay around given setpoints by properly adjusting the inspiratory oxygen fraction fiO2 at the ventilator. In fact, if starting from a low level, the airway pressure is increased continuosly, the fiO2 required to keep SO2 constant will decrease while this fiO2 will increase with a reduction of airway pressure.
For an automatic detection of alveolar opening during a continuous increase of airway pressure, one possibilities is to look for the negative maximum of the gradient of fiO2 set by the controller. Similarily, an alveolar closing may be identified by detecting the positive maximum of the gradient of fiO2 set by the controller during a continuous decrease in airway pressure.
Another related object of this invention is to provide a method in which the endtidal CO2 concentration in the exhaled air flow is measured which can be used to detect alveolar opening or closing of the lung.
In addition or instead, the CO2 output from the body may also be measured and used for the same task. Note that the CO2 output ([ml CO2/min]) may be obtained by continuously measuring the CO2 concentration in the expired air as well as the air flow and the respiratory rate.
When the airway pressure is changed during ventilation, the endtidal CO2 concentration and the CO2 output behave similar. Thus, if the airway pressure is increased starting from a low value, the endtidal CO2 concentration and the CO2 output also increase. If the airway pressure is decreased afterwards, the endtidal CO2 concentration and the CO2 output decrease as well.
Within an automatic signal monitoring device, a criterion for alveolar opening can be to e.g. look for a maximal change in the positive gradient of either the endtidal CO2 concentration or the CO2 output when simulataneously increasing the airway pressure continuously starting from a low value. In other words, during a continuous pressure rise alveolar opening occurs when the second time derivative of etCO2 and/or of CO2 output has a maximum while the first derivative is positive.
Similarily, a criterion for alveolar collapse can be e.g. a maximal change in the negative gradient of either the endtidal CO2 concentration or the CO2 output during a continuous decrease in airway pressure. In other words, alveolar closing occurs during a continuous decrease in airway pressure if the second time derivative of etCO2 and/or of CO2 output has a minimum while the first derivative is negative.
In a preferred embodiment of this invention, the methods for identification of alveolar opening or collapse pressures as claimed in this invention may be used to build a device for protective artificial ventilation in which a central processing unit (CPU) uses the identified opening and closing pressures to automatically set at least one ventilation parameter of an artificial ventilator such that a maximal gas exchange can be achieved while simultaneously minimizing mechanical stress on lung tissue.